Nuclear magnetic resonance (NMR) is a physical phenomenon in which certain atomic nuclei in the presence of an applied, external magnetic field absorb and re-emit electromagnetic radiation. This phenomenon is harnessed in NMR spectroscopy, one of the most powerful analytical techniques for determining the composition and structure of materials. In NMR spectroscopy, radio frequency (RE) pulses are irradiated into a sample of material positioned in a strong, static magnetic field, followed by measurement of the electromagnetic response of the sample. This measurement is used to generate a spectrum of one or more lines representing the resonant frequency or frequencies of the target nucleus (e.g., 1H) relative to a standard (i.e., the chemical shift(s)). The position, number, and size of chemical shifts are indicative of the relative positions/electronic environments of the target nuclei in a material and are diagnostic of the structure of the material.
NMR spectroscopy has been more successfully used in the analysis of liquids or materials dissolved in solvents than of solids. The basic problem in NMR spectroscopy of solids is that the rapid molecular tumbling and diffusion (i.e., Brownian motion) present in liquids and solutions that averages out anisotropic dipole coupling interactions and chemical shift anisotropy (CSA), the main causes of line broadening in NMR spectroscopy, is not present in solids. Thus, the lines in NMR spectra of solid samples are typically broad and unresolved, oftentimes tens to hundreds of ppm in width. In many instances, the line broadening is so severe that lines having different chemical shifts cannot be readily discerned from each other.
“Magic Angle” Spinning (MAS) is the most widely used technique developed to attenuate the line broadening that occurs from anisotropic dipole coupling interactions and CSA during NMR spectroscopy of solids. While not true isotropic motion, MAS involves spinning the sample extremely rapidly along a fixed axis at the “Magic Angle” (i.e., 54.74°) with respect to the direction of the externally applied magnetic field, B0. Complicated multiple pulse programs are another approach used, often in conjunction with MAS (i.e., combined rotation and multiple-pulse spectroscopy—CRAMPS), to decouple anisotropic dipole coupling interactions and CSA. However, while substantial improvements in spectrum resolution are generally obtained from using MAS NMR, either alone or in the combination with multiple-pulse programs, these techniques have so far been unable to generate NMR spectra having resolutions comparable to those seen in spectra of liquids or solutions.
Decoupling of anisotropic dipole coupling interactions using MAS requires, at a minimum, that the spinning frequency of the sample be higher than the homonuclear dipolar coupling frequency of the target nuclei. This presents substantial technological challenges, especially when the homonuclear dipolar coupling frequency is very large. For example, since the dipolar coupling of 1H is greater than 100 kHz, its decoupling would require that the sample container be spun at an even higher spinning frequency. Likewise for CSA. While most CSA effects can be effectively attenuated by MAS at frequencies of from 6 to 10 kHz, CSA effects can be more pronounced in stronger magnetic fields, thus requiring even higher MAS spinning frequencies. However, very few MAS NMR spectrometers are technically capable of such ultra-fast spinning frequencies, with most spectrometers only capable of spinning frequencies of 20 to 25 kHz or less. Moreover, even those that have such capability still produce NMR spectra of inferior resolution to those produced from the analysis of samples in a liquid or solvated phase.